Basically, Silver concludes that Ryan was a risky choice and that Romney would not have made such a risky pick unless he thought he was down in the race and therefore had little to lose.

(Silver agrees with this assessment, giving Romney only a 30% chance of winning. He also compliments Romney on recognizing the reality of his situation, as opposed to believing his own stump speeches the way other politicians do.)

Why is Ryan such a risky pick?

Because "Politics 101," Silver says, dictates that VP candidates that are closest to the center, ideologically speaking, give the Presidential candidate the best chance of winning.

And Ryan is the most extreme VP candidate since at least 1900, Silver says—on either side of the aisle.

Silver includes a chart using the statistical system "DW-Nominate." This system concludes that Paul Ryan is about as conservative as Michele Bachmann, and farther from the center than any other VP candidate in the past century.

In other words, Silver concludes, Ryan is Hail Mary pass for Mitt Romney. But when you're down big late in the fourth quarter, as Romney is, you might as well throw the ball. Because sometimes the receivers haul it in and you win.